The present invention relates generally to switching regulators, one type of power source devices, used for a facsimile apparatus, a printer or the like, and more particularly to a switching regulator of a ringing choke converter (called R.C.C for short) type. Hereupon, the R.C.C controls output voltage by a self-oscillator which oscillates between ON and OFF states.
A description will now be given of an operational principal of a conventional R.C.C. The conventional R.C.C, as shown in FIG. 1, comprises transistors 11 and 12, a rectifying diode 13, an comparator 14, a transformer 15 having a primary winding 16, a secondary winding 17, and a bias winding 18, a smoothing capacitor 19 and a zener diode 20. The transistor 11 oscillates between the ON and OFF states because of the positive feedback connection with the primary and bias windings 16 and 18. The input DC voltage is converted into rectangular voltage at the secondary winding 17, and is reconverted into the DC voltage by the smoothing capacitor 19. During this course, energy is charged to a transformer 15 while the transistor is in the ON state, and discharged therefrom to a load via the rectifying diode 13 when the transistor 11 changes to the OFF state. The transistor 12, comparator 14, and zener diode 20 control the ON state frequency of the transistor 21. The load is to be connected to the output terminal of the R.C.C. If the level of the load becomes relatively large, the output voltage is getting slightly small. And if the level of the load becomes relatively small, vice versa. The comparator 14 compares the level of output voltage with the level of constant voltage of the zener diode 20. If the level of output voltage is larger than the level of the constant voltage of the zener diode 20, the transistor 12 is set in the ON state because of a signal from the comparator 14. The load current decreases when the load becomes large, and the load current increases when the load becomes small. Hereinafter, a description will be given based on the assumption that the load current changes as shown in FIG. 2(a). Collector current of the transistor 11 changes accordingly, as shown in FIG. 2(b). According to FIG. 2, the higher a load current is, the higher peaks of the collector current of the transistor 11 is, and the lower the oscillation frequency thereof is. And the lower the load current is, vice versa. Incidentally, other features of the conventional R.C.C are explained in detail in "SWITCHING REGULATOR SEKKEI JISSOU OYOU MANUAL" published by Japanese Industrial Technology, pp. 80-85", for example. Thus, as shown in FIG. 2, when the load current is low, the conventional R.C.C oscillates incessantly, causing an instable circuit operation thereof, a high noise frequency, and a low conversion efficiency because of a switching loss.